1. Field of the Invention
The present invention relates to a waveform information display apparatus such as a digital oscilloscope, and particularly relates to a waveform information display apparatus for displaying display data on which the property of captured waveform data is reflected.
2. Related Art
In analog oscilloscopes, a waveform based on an analog input signal is displayed continuously on a cathode ray tube (CRT) having a certain persistence characteristic. Therefore, the frequency information of the waveform is displayed as brightness on the screen. However, in the analog oscilloscopes, all processings are carried out in real time, and the displayed data is regarded as transient and is not stored internally. Thus, the data displayed on the CRT disappears when persistent time has passed.
It is therefore difficult to measure and display any property of the waveform such as a frequency, a period, a maximum value or a minimum value except the waveform itself. Indeed, in some analog oscilloscopes, not only a waveform but also its frequency, period, maximum value, minimum value, etc. are measured and displayed by analog processing. There is however room for improvement in displaying the properties of the wave form concurrently with the waveform itself.
On the other hand, in digital oscilloscopes, an analog input signal is sampled at a regular time interval and converted into digital waveform data. The converted waveform data is stored in an internal memory. Then, a waveform corresponding to the analog input signal is displayed on a raster scanning display unit or the like on the basis of the digital waveform data stored in the internal memory.
In such a manner, in the digital oscilloscopes, since the digital waveform data is stored in the internal memory, various arithmetic processings can be carried out on the stored digital waveform data, and the results of the processings can be displayed concurrently on the screen displaying the waveform.
However, the display in the digital oscilloscopes has the following problems.
In the raster scanning display unit typically used in the digital oscilloscopes, the display resolving power of a normal display screen in the direction of the time axis is only about 1,000 points. Accordingly, in order to match a plurality of pieces of digital waveform data of 1,000 or more points with the display resolving power of about 1,000 points, the digital waveform data is divided into time segments each having a predetermined length corresponding to the display resolving power. Data in the same time segment is compressed to generate compressed display data, and a compressed waveform is displayed on the basis of the compressed display data.
As for the method for displaying the compressed waveform, there is a method in which a maximum value and a minimum value of data in the same time segment are obtained, connected in the vertical axis direction and displayed, or a method in which all pieces of data in the same time segment are displayed on a specific line in the vertical axis direction.
However, in such a compressed waveform display method for digital oscilloscopes, waveform frequency information cannot be expressed on the screen as perfectly as that in analog oscilloscopes. Accordingly, there is another display method in which the frequency of occurrence of each piece of data in the vertical axis direction is obtained from the data in the same time segment, and identification (for example, identification by brightness, density, color or pattern corresponding to the frequency of occurrence) is applied to display data in accordance with the frequency of occurrence frequency of the display data.
In addition, the description has been made on the assumption that waveform data captured in certain time series is displayed whenever the waveform data is captured. However, display like an analog oscilloscope can be attained if the accumulated frequency of occurrence of repeatedly captured data in the same time segment is obtained, and identification using brightness, density, color or pattern by way of example is applied to display data in accordance with the frequency of occurrence of the display data. Nowadays, digital oscilloscopes in which these functions are incorporated to enable display like an analog oscilloscope have also been produced on a commercial basis.
In such a manner, the method for applying identification (brightness, density, color or pattern) to the display of a digital oscilloscope in accordance with the frequency of occurrence of data is chiefly intended to make the display of the digital oscilloscope more approach to the display of an analog oscilloscope. The method becomes effective when the number of elements composing a waveform contained in one time segment is not very large.
However, with the increase of the memory capacity, the number of points of digital waveform data captured at one time may increase to about several tens of megas, and the number of elements composing a waveform contained in one time segment may increase. In such a case, it will be difficult to grasp the outline of the waveform correctly by only applying the frequency of occurrence to display data.
Further, various functions for arithmetically processing the stored digital waveform data are indeed attained in digital oscilloscopes. For example, an automatic measuring function for measuring the pulse width of a positive-direction pulse (hereinafter, referred to as “+ pulse width”) has been attained. However, in the prior art, the result of such an arithmetic processing is not added directly to the waveform data but is provided in a display form different from the display form of the waveform data. For example, the arithmetic processing result is displayed as numeric data or displayed as a trend waveform (in which an event and a result for grasping a trend are displayed two-dimensionally) or as a histogram.
When the arithmetic processing result is provided as numeric data, it is difficult to grasp the property of a waveform by the light of nature. On the other hand, when the arithmetic processing result is provided as a trend waveform or a histogram, it is possible to grasp the property of the waveform by the light of nature but it is difficult to grasp the relationship with the compressed display waveform correctly. Further, it is necessary to perform the display of the waveform itself and the display of such numeric data, a trend waveform or a histogram concurrently. Therefore, there also arises a problem that effective use of a waveform display area becomes difficult when it is desired to use the wave form display area for another purpose. For example, when a large volume of digital waveform data is observed, an entire waveform and a partial waveform thereof zoomed in on (zoomed waveform) are often displayed concurrently. It is therefore required a waveform information display method in which the property of a displayed waveform can be grasped by the light of nature without displaying any extra display other than the display of the waveform itself.
In addition, a zoomed waveform showing an enlarged part of the entire waveform is used for observing the details of the waveform data. However, when the number of points of digital waveform data captured at one time reaches about several tens of megas with the increase of the memory capacity, it is necessary to check several thousands of screens of waveform data in order to check the entire waveform in a horizontal axis range (time axis range) with which the details of the waveform can be checked. Thus, it becomes quite difficult to attain such check.
Most observers do not intend to observe all the waveform data but intend to examine presence/existence of a glitch or to check the variation of a period or an amplitude. In this case, the observers carry out observation in the following manner. That is, the observers visually extract a portion having a desired property from the entire waveform or from the waveform data in which the entire waveform has been zoomed in on several times. Then, the observers zoom in on the extracted portion manually to an extent that the zoomed portion is large enough to make the observers identify the details of the portion. In order to observe waveform data more easily, there are required measures for extracting a specified property from waveform data so as to facilitate identification of the details easily.
In consideration of these facts, an object of the present invention is to provide a waveform information display apparatus such as a digital oscilloscope as described above, in which the property of waveform data can be seen with ease and by the light of nature even if the details of the waveform are hard to observe directly due to compressed display.
In addition, another object of the invention is to provide a waveform information display apparatus in which the details of a waveform displayed in compression can be observed easily.